1. Field of the Invention
The present invention is directed to polyisocyanates stabilized with certain chlorine-substituted carboxylic acids.
2. Description of the Prior Art
The polyisocyanates used for lacquers and coatings are generally reaction products of diisocyanates. The diisocyanates are modified by urethanization, urea formation, allophanatization, biuretization, trimerization, dimerization and similar reactions. The polyisocyanates thus modified often contain very little, if any, unreacted starting diisocyanate. However, they contain highly reactive NCO groups which are to be used for subsequent applications. These NCO groups have to remain stable and unreacted for prolonged periods during processing.
It has already been proposed to stabilize modified polyisocyanates by the addition of certain compounds that are generally acidic in character. A stabilizer has to perform several functions, i.e., it has to bind and deactivate reaction accelerators and catalysts which may have been used in previous reactions or which may be unintentionally contained in reactants such as polyethers or polyesters; it has to prevent the polyisocyanate from being adversely affected by light or similar outside influences and must not itself produce any changes such as discoloration; it has to ensure that the polyisocyanate remains stable and retains its high reactivity; it has to ensure that the reactivity of the polyisocyanate to reagents such as polyalcohols or optionally blocked polyamines is reproducible; and it must also be a catalyst for these reactions.
It follows from this that in the context of the present invention, "stabilizers" are not the polyurethane foam stabilizers which have long played an important part in isocyanate chemistry and are not surface-active agents, for example based on polysiloxanes.
The synthesis of polyisocyanates which are suitable for coating purposes or which are useful for adhesives and sealing compounds has been described in numerous publications, of which the following are cited as examples:
1. G. W. Becker and D. Braun, Kunststoff Handbuch, Vol. 7 "Polyurethane", edited by G. Oertel, Carl-Hanser-Verlag, Munchen, Wien 1983, pages 540-610.
2. J. H. Saunders and K. C. Frisch, "High Polymers", Interscience Publishers, New York 1962, Vol. XVI
3. H. Kittel, Lehrbuch der Lacke und Beschichtungen, Verlag W. A. Colomb Berlin 1973, Vol. I, Part 2
4. K. C. Frisch, P. Kordomenos, American Chemical Society Symposium, 285, "Applied Polymer Science", page 985, 2nd Edition, Washington 1985.
The modification of polyisocyanates used for coating compositions, sealing compounds and adhesives by the addition of certain acids or acidic substances for the purpose of stabilization and/or control of reactivity with respect to certain reactants, such as polyols or optionally blocked polyamines, has been described in several publications. According to EP-A 155,559, for example, basic catalysts present in isocyanurate polyisocyanates are neutralized by acids such as phosphoric acid, dodecylbenzene sulfonic acid, monochloroacetic acid, monofluoroacetic acid or benzoyl chloride, and the polyisocyanates are thus stabilized. According to EP-A 239,834 and EP-A 254,177, compounds such as formic acid, acetic acid, mono-, di- and trichloroacetic acid, oxalic acid, malonic acid, maleic acid, fumaric acid, benzoic acid, mono-, di- and trichlorobenzoic acid, salicylic acid, toluenesulfonic acid, xylenesulfonic acid, etc., are added to polyisocyanates containing urethane groups. The effect of this addition is that the polyisocyanates mentioned show constant reactivity to reactants such as polyaldimines, and react with these reactants in the presence of moisture. The additives also influence the hydrolysis rate of the polyaldimines.
The known additives mentioned above only satisfy some of the above-stated requirements for a stabilizer. Some of them even have adverse effects and inter alia discolor the polyisocyanate or impair the stability of the coating compositions, sealing compounds and adhesives in the presence of hydrolytic influences. Their effect can diminish during storage so that the properties of the polyisocyanates to be stabilized can change. Because of this, it has also been proposed to additionally use other stabilizers such as phenothiazines, sterically hindered phenols, etc. (Cf. EP-A 239,834).
Accordingly, an object of the present invention is to provide modified polyisocyanates showing improved stability in storage. This object has surprisingly been achieved by the effectiveness of special carboxylic acids as stabilizers in accordance with the present invention.